Many fields of industry are now using fiber optic sensors (herein abbreviated FOS) for applications that require sensing over long ranges. This includes distributed acoustic sensing systems, temperature sensing systems, leak detection systems, systems for monitoring the structural stability of bridges, buildings, railroads and the like. Long range fiber optic sensing applications can include pipeline sensing systems for determining the precise location of leaks in pipelines carrying liquids and gases such as gasoline and water, as well as border detection systems for detecting intrusions over a border.
Fiber optic sensing systems utilize fiber optic cables (also known as and referred to as optical fibers or sensing fibers) to carry information over long distances from sensors placed along the fiber optic cables. The sensors may be point sensors, fiber Bragg gratings (herein abbreviated FBG) and the like. The information carried is essentially light and characteristics related to light which is provided to a processor for processing the information. Differences in the characteristics of the light reaching the processor, such as FBG reflection amplitudes or resonance shifts, can be used to measure temperature or strain, for example. In other fiber optic sensing systems, instead of sensing light from discrete sensors, information is carried by the scattering intensity, resonances or interference patterns of light scattered from the entire length of an optical fiber. In this sense, the fiber optic sensing system is a distributed sensing system. In this type of sensing system, the entire optical fiber can serve as a continuous array of sensors or a distributed fiber optic sensor (herein abbreviated DFOS). In DFOS systems, different known physical scattering phenomena such as Rayleigh scattering, Raman scattering and Brillouin scattering can be utilized to produce signal light which can be measured for sensing purposes. For example, a map of disturbances or phase changes in the Rayleigh back-scattering pattern along the sensing fiber can be generated and utilized to detect a gas leak in a pipe, an intruder, the presence of fire in a tunnel, and the like. Alternatively, a physical parameter (such as temperature, strain, pressure and the like) can be determined by changes in one of the above mentioned scattering phenomena along the sensing fiber. A map of the temperature along the sensing fiber can be generated by measuring the shift in the Brillouin scattering resonance of either forward-scattered or back-scattered light, or by measuring the relative amplitudes of Raman scattering along the sensing fiber. In DFOS systems, the received signal must be correlated to the location of the scattering event in the sensing fiber thus enabling the determination of the location along the sensing fiber. This can be done by using probe pulses and analyzing the scattered signal in the time domain. In the case of back-scattered signals this method is known as optical time domain reflectometry (herein abbreviated as OTDR). In cases where the forward-scattered signal is analyzed, the method is known as optical time domain analysis (herein abbreviated as OTDA). An alternative way of correlating the signal to the location in the sensing fiber is by sweeping the laser signal and analyzing the measured signal in the frequency domain, as is done for example in a method referred to as optical frequency domain reflectometry (herein abbreviated as OFDR).
Fiber optic sensing systems in which the forward-scattered signal is analyzed such as OTDA must allow access to the sensing fiber at both ends, either enabling a loop geometry or having two distinct control centers at both fiber ends, each requiring electricity and communication between them. An example of such a sensing system is a Brillouin OTDA system (herein abbreviated BOTDA). Fiber optic sensing systems in which back-scattered signals are measured can be designed so that the system requires access to only one end of the sensing fiber, wherein the probe pulse or light beam is both transmitted and received from a first control center. These kinds of systems are useful in scenarios where the second end of the optical fiber is either inaccessible or difficult to access. Examples of such scenarios include pipelines, deep drilling systems, borders and roads wherein it is either impossible, problematic or expensive to lay the necessary infrastructure (such as electricity and communication systems) at the second end of an optical fiber. It is noted as well in that such configurations, the distance between control centers that require electricity and communication systems can be twice the sensing fiber length provided that access to both ends of the optical fiber is not required.
The practical length of the sensing fiber is limited by weak back-scattered and/or forward-scattered signals as well as by attenuation of the probe light or pulse and attenuation of the scattered signals. The signal-to-noise ratio (herein abbreviated SNR) of the sensed scattered signal must be high enough to differentiate the sensed signal from noise in the signal. For weak enough signals, shot-noise is the dominant source of noise, causing the SNR to decrease with the measured power. Typical fiber optic sensing systems can provide signals up to 20-50 kilometers before the attenuation is significant enough to make signal detection difficult. One solution for increasing the length of detection of such a fiber optic sensing system would be to place an optical amplifier every 40-50 kilometers such that the signal can travel distances of hundreds of kilometers if necessary. However, such a solution has limitations. It requires access to the sensing fiber, which may be buried underground for example in the case of a buried pipeline or a border breach detection system. Moreover, it requires the infrastructure of electricity and communications at every amplification site. In addition, these sites might need protection against tampering and/or sabotage thus increasing the maintenance costs of such a system. The cost of the setup and maintenance of such an array of amplifiers render this solution impractical for many scenarios.
Another solution to increasing the reach of an FOS system would be to simply send an initial probe pulse or light beam with higher power. However non-linear effects and phenomena such as stimulated Raman scattering (herein abbreviated SRS), stimulated Brillouin scattering (herein abbreviated SBS) and modulation instability (herein abbreviated MI), limit the power of the probe pulse or light signal to a range of single milliwatts to hundreds of milliwatts depending on the application and configuration of the FOS system. Powers above that range will result in non-linear effects in the sensing fiber, thereby preventing accurate detection of any scattered signals. The power range above which non-linear effects and phenomena begin to occur is referred to herein as the non-linear threshold.
Reference is now made to FIG. 1, which is a schematic illustration of a fiber optic sensing system with no additional amplification, generally referenced 10, as is known in the art. Fiber optic sensing system 10 is a phase OTDR system using direct detection of back-scattered light from a sensing fiber wherein the end of the fiber is inaccessible to electronics or optical pumping. Fiber optic sensing system 10 includes an interrogation unit 26 which is coupled to a sensing optical fiber 22 (also referred to as a sensing fiber, an optical fiber or simply a fiber). Interrogation unit 26 includes a narrow linewidth probe laser 12, a modulator 14, a circulator 16, a detector 18 and a processor 20. Modulator 14 is coupled with probe laser 12 and circulator 16. Detector 18 is coupled with processor 20 and is optically coupled with circulator 16 to receive and detect light coming from circulator 16. Optical fiber 22 is coupled with circulator 16 and has an end point 24. For the purposes of simplicity, any amplification stages which might be used in fiber optic sensing system 10 either for amplifying probe light generated anywhere between probe laser 12 and sensing optical fiber 22 or for amplifying back-scattered light anywhere between sensing optical fiber 22 and detector 18 are omitted.
Probe laser 12 provides probe light which is modulated by modulator 14 to create a probe pulse. The probe pulse is sent to circulator 16 which sends the probe pulse along the length of optical fiber 22. As the probe pulse travels down optical fiber 22, it is partially back-scattered towards circulator 16. The back-scattered light is separated from the probe pulse by circulator 16 which can be an optical circulator or optical splitter.
The back-scattered light is provided by circulator 16 to detector 18, which detects the characteristics of the light and provides the detected characteristics to processor 20. The processor compares the interference patterns generated by different pulses and correlates the time of the received signal to the location of its reflection along sensing fiber 22. Provided the SNR of the probe pulse is high enough, processor 20 can determine at what distance from interrogation unit 26 a significant enough change occurred in the phase of the probe pulse which might indicate a leak, an intruder and the like.
Characteristics about the back-scattered light are captured by detector 18 and are provided to processor 20 for extracting information about the back-scattered light. Examples of such characteristics can include frequency, phase, intensity, interference and the like. In FIG. 1 the spatial resolution is determined by the pulse width generated by modulator 14. The spatial resolution refers to the resolution in distance wherein a change in a measurand such as temperature or strain can be determined and practically refers to the length of fiber in which a leak or an intruder can be located. To ensure that there is no overlap between the back-scatterings of consecutive probe pulses, the pulse repetition rate (herein abbreviated PRR) of the probe laser must be lower than the inverse of the roundtrip time it takes the probe pulse to propagate along optical fiber 22 to end point 24 and back to circulator 16. As mentioned above, the peak power of the probe pulse must be limited to avoid non-linear phenomena occurring along optical fiber 22.
A practical use of fiber optic sensing system 10 is the detection of an intruder over a border protection system using Rayleigh scattering. In order to avoid non-linear phenomena in optical fiber 22, it is assumed that the peak power entering sensing fiber 22 is limited to 5 milliwatts (herein abbreviated mW). The probe pulse provided by probe laser 12 propagates down optical fiber 22 and any light which is back-scattered along optical fiber 22 due to Rayleigh scattering is received by circulator 16 and provided to detector 18. The intensity of the back-scattered light has a speckle-like pattern resulting from interference between different scattering events that occur within the probe pulse at different locations along optical fiber 22, reaching detector 18 simultaneously. Any local change in the index of refraction will change the phases between back-scattered constituents of the signal and will result in a change in the speckle-like pattern of consecutive probe pulses. Such a change in the back-scattered probe light measurement is indicative of an intruder and since the roundtrip time of the probe light which caused the change is known, the location of the breach of the intruder along the border can be determined.
Reference is now made to FIG. 2, which is a graph of a simulation using the fiber optic sensing system of FIG. 1, showing the absolute value of the difference between two consecutive probe light measurements on a logarithmic scale, generally referenced 30, as is known in the art. Graph 30 includes an X-axis 32, showing distance in kilometers (herein abbreviated km) along a sensing fiber (such as optical fiber 22 in FIG. 1) and a Y-axis 34, showing the power in decibels referenced to one milliwatt (herein abbreviated dBm). The simulation as shown has been designed to exhibit disturbances between measurements along the optical fiber at the following distances: 15 km, 40 km, 65 km, 90 km and 115 km. In a sensing system as in FIG. 1, shot-noise is a dominant source of noise and is shown in graph 30 via an arrow 40. As can be seen by an arrow 36, there is a natural quasi-linear attenuation of about 0.2 decibels (herein abbreviated dB) per kilometer in the noise level. This is a result of an attenuation of about 0.4 dB/km in the back-scattered signal (0.2 dB/km in each direction of the roundtrip) and the square root dependence of shot noise power on the signal power. A first disturbance is shown at 15 km via an arrow 38 whereas a second disturbance is shown at 40 km via an arrow 42. In the simulation, the pulse width was set to 100 nanoseconds (herein abbreviated ns) which corresponds to a spatial resolution of about 10 meters. For the selected parameters, as shown, the first disturbance at 15 km (arrow 38) is easily detected by a processor however the second disturbance at 40 km (arrow 42) is nearly impossible to detect given the attenuation along the optical fiber. By 40 km, the shot noise is as strong at the disturbance, which is not resolvable unless some form of in-line amplification is used to boost the disturbance signal over the noise signal. The disturbances at 65 km, 90 km and 115 km are drowned out by noise. Thus the sensing system as shown in FIG. 1 may be good for applications in which a reach of up to 40 km is desired however for distances larger than that, the sensing system of FIG. 1 is inadequate. Such sensing systems are known in the prior art and are described, for example, in Grattan, K. T. V. and Sun. T., “Fiber optic sensor technology: an overview,” Sensors and Actuators A Physical, volume 82, numbers 1-3, May 2000, pp. 40-61 and in Santos, J. L. and Farahi, F., Handbook of Optical Sensors, CRC Press, Boca Raton, 2015.
As shown and explained above in FIGS. 1 and 2, the power of the captured back-scattered light pulse or signal by the detector is typically a small fraction of the power of the probe pulse, which itself is limited. In the case of fiber optic sensing systems using fiber Bragg gratings (herein abbreviated FBGs), the power of the captured back-scattered signal may be than less 10% of the power of the probe pulse, whereas in DFOS systems, the power of the captured back-scattered signal may be less than 10−3% of the power of the probe pulse. Add to this the natural attenuation of both the probe pulse and the back-scattered light pulse thereby makes the application of DFOS systems over tens of kilometers in length non-trivial. As shown in FIG. 2, a low SNR of the back-scattered light pulse can limit the spatial resolution of a sensing system, leading to high false alarm rates, incorrect classification or missed events in an intrusion detection system and also resulting in low accuracy of the measurand, such as strain, temperature, flow and/or pressure in other sensing applications.
As mentioned above, one way of extending the reach of a fiber optic sensing system, for example to extend the reach of the system of FIG. 1 as simulated in FIG. 2, is to place amplifiers along the optical fiber for overcoming attenuation within the fiber. Such an approach is known in the art and is described in Lai, M. et al., “Ultra-long Distance Distributed Intrusion Detecting System Assisted With In-line Amplification,” IEEE Photonics Journal, volume 9, number 2, April 2017, pp. 1-10. Whereas this approach may increase the reach of a fiber optic sensing system, it requires bringing electricity and communications to multiple control centers or control points along the optical fiber. This may involve bringing pump light to additional amplifiers positioned along the optical fiber and also adding maintenance points, thus losing some of the advantages of long distance FOS.
One approach to increasing the reach of a fiber optic sensing system without the use of external amplifiers is to use in-line Raman amplification of the probe pulse, the back-scattered signal or both. Reference is now made to FIG. 3, which is a schematic illustration of a fiber optic sensing system using in-line Raman amplification, generally referenced 50, as is known in the art. Fiber optic sensing system 50 includes an interrogation unit 70, which is coupled to a sensing optical fiber 68. Interrogation unit 70 includes a narrow linewidth probe laser 52, a modulator 54, a circulator 56, a detector 58, a processor 60, a wavelength division multiplexer (herein abbreviated WDM) filter 62 and a Raman pump laser 64. Modulator 54 is coupled with probe laser 52 and circulator 56. Detector 58 is coupled with processor 60 and is optically coupled with circulator 56 to receive and detect light coming from circulator 56. WDM filter 62 couples light from the Raman pump laser 64 to the sensing fiber 68. Optical fiber 68 is coupled with circulator 56 through WDM filter 62 and has an end point 66. For the purposes of simplicity, any amplification stages which might be used in fiber optic sensing system 50 either for amplifying a probe light anywhere between probe laser 52 and sensing optical fiber 68 or for amplifying back-scattered light between sensing optical fiber 68 and detector 58 are omitted.
The sensing system shown in FIG. 3 operates in a manner similar to the sensing system shown in FIG. 1 however it differs in that fiber optic sensing system 50 employs first order Raman amplification. Raman pump laser 64 produces pump light which is combined with the probe pulse produced by modulator 54 (from light generated by probe laser 52). The pump light is combined with optical fiber 68 via WDM filter 62 and can provide in-line amplification both to the probe pulse travelling down optical fiber 68 and the back-scattered light generated in sensing fiber 68 and propagating towards circulator 56. In general, the wavelength of the pump light generated by Raman pump laser 64 is selected such that it is near the peak of the Raman scattering spectrum. Like the probe pulse, the pump light provided by Raman pump laser 64 is also attenuated as it propagates down optical fiber 68. The pump light attenuates and decays exponentially along the distance of optical fiber 68. Therefore the maximal Raman gain is at the end of optical fiber 68 nearest to interrogator unit 70. The power of Raman pump laser 64 must be limited to avoid unwanted non-linear phenomena such as spontaneous Raman scattering occurring along optical fiber 68 which can mask any back-scattered signals and deteriorate the signal SNR. The Raman laser pump power is thus kept below a threshold for spontaneous Raman scattering at all locations along optical fiber 68. Attenuation of the pump light as it propagates along optical fiber 68 limits the in-line amplification of the probe pulse to a distance which is proportional to the attenuation length of the wavelength of Raman pump laser 64. This is typically around 10 km.
As mentioned above, the light generated by Raman pump laser 64 amplifies both the back-scattered light and the probe pulse itself. In order to keep the probe pulse power below the threshold for non-linear phenomena to be exhibited, either the Raman laser power that overlaps with the probe pulse must be limited to achieve very low amplification or the power of the probe pulse must be limited, which ultimately may reduce the SNR of the probe pulse. Therefore, when modulator 54 generates a probe pulse, the power of Raman pump laser 64 can be lowered or even set to zero.
In a variation of fiber optic sensor system 50, the pump light generated by the Raman pump laser can be coupled to the end of the sensing fiber and counter-propagated to the probe pulse generated by the probe laser (not shown in FIG. 3).
Reference is now made to FIG. 4, which is a graph of a simulation using the fiber optic sensing system of FIG. 3, showing the absolute value of the difference between two consecutive probe light measurements on a logarithmic scale, generally referenced 80, as is known in the art. Graph 80 includes an X-axis 82, showing distance in kilometers along an optical fiber (for example, sensing fiber 68 from FIG. 3), and a Y-axis 84, showing the power in dBm. The simulation as shown has been designed to exhibit disturbances between measurements along the optical fiber at the following distances: 15 km, 40 km, 65 km, 90 km and 115 km. Regarding the parameters of the sensor system of FIG. 3, the Raman pump laser is set to zero when the probe pulses are generated. At all other times, the power of the Raman laser is at 400 mW, which is just below the threshold for spontaneous Raman scattering. All other parameters as described above regarding FIG. 2 are the same. Shot-noise is shown via an arrow 90. The Raman pump light and the gain it supplies to the back-scattered light is exponentially attenuated as it propagates down the fiber. From relatively close locations, for example up to around 17 km, this gain is larger than the attenuation of the probe pulse and the back-scattered light, so the returned signal from this region is actually larger than locations further along the sensing fiber. Since both shot-noise 90 and the SNR grow with power, initially both shot-noise 90 and the back-scattered light SNR increase, shown as a section 86A. After around approximately 17 km, the amplification provided is less than the combined attenuation of the probe pulse and the back-scattered light. Thus after around 20 km, there is negligible in-line Raman amplification and the decay rate of the signal, shown as a section 86B, is similar to that of FIG. 2. Three separate arrows 88A, 88B and 88C respectively show the received power of the disturbances at 15 km, 40 km and 65 km respectively. As shown in the graph, the distance along the optical fiber which produces the strongest probe pulse signal is around 20 km. The disturbances at 15 km and 40 km are clearly visible and discernible, whereas the disturbance at 65 km, shown by arrow 88C, is comparable in power to the shot noise power. The disturbances at 90 km and 115 km however are masked by the shot noise and are not detectable using the sensor system of FIG. 3. The sensor system of FIG. 3 improves the reach of an FOS over the sensor system of FIG. 1, however the useable distances of such a sensor system are still limited.
Sensing systems as described in FIG. 3 are known in the art and are described in, for example, U.S. Pat. No. 8,989,526, issued to Hartog, entitled “Raman Amplification in Distributed Optical Fiber Sensing Systems” and Martins, H. F. et al., “Phase-sensitive Optical Time Domain Reflectometer Assisted by First-order Raman Amplification for Distributed Vibration Sensing Over >100 km,” Journal of Lightwave Technology, volume 32, number 8, April 2014, pp. 1510-1518.
The limitations of the sensor system of FIG. 3 can be partially overcome by using a sensor system employing second-order Raman amplification. Such a sensor system is shown in FIG. 5. Reference is now made to FIG. 5, which is a schematic illustration of a fiber optic sensing system using second order in-line Raman amplification, generally referenced 100, as is known in the art. Fiber optic sensing system 100 includes an interrogation unit 102 and an optical sensing fiber 126. Interrogation unit 102 includes a probe laser 106, a modulator 108, a circulator 110, a detector 112, a processor 114, a first WDM filter 116, a Raman laser 118, a first FBG 120A, a second FBG 120B and a second WDM filter 122. Interrogation unit 102 is coupled with optical fiber 126 at both ends, thereby forming an optical fiber loop 124. Second WDM 122 is coupled with Raman laser 118 and second FBG 120B couples second WDM 122 with optical fiber 126. Modulator 108 is coupled with probe laser 106 and circulator 110. Detector 112 is coupled with processor 114 and is optically coupled with circulator 110 to receive and detect signals coming from circulator 110. First WDM filter 116 is coupled with circulator 110, Raman laser 118 and first FBG 120A. Interrogation unit 102 substantially represents a respective control center which requires power in order to operate the probe laser and the Raman laser. Raman laser 118 can provide a forward propagating pump light via first WDM 116 and first FBG 120A, a counter-propagating pump light via second WDM 122 and second FBG 120B or both. For the purposes of simplicity, any amplification stages which might be used in fiber optic sensing system 100 for amplifying a probe light anywhere between probe laser 106 and optical fiber 126 or for amplifying back-scattered light between optical fiber 126 and detector 112 are omitted.
The sensor system shown in FIG. 5 is an example of a counter-propagating pump light, as Raman laser 118 can provide a counter-propagating Raman pump light via second WDM 122. The sensor system of FIG. 5 however requires that both ends of optical fiber 126 be accessible by interrogator unit 102, only enabling a loop geometry for sensing fiber 126 (i.e., optical fiber loop 124). The maximal reach along the sensing fiber for measurements is then only half the length of sensing fiber 126 due to the loop geometry. Such a sensor system thus cannot be used in scenarios where one fiber end is to be free and inaccessible and also halves the maximal distance between control points in long distance scenarios. In scenarios where sensing fiber 126 does form a loop, it is possible to use a system similar to fiber optic sensing system 50 (FIG. 3) with two free-ended sensing fibers (each going in an opposite direction), thereby forming a loop with a total length of twice the length of a single sensing fiber (as shown in FIG. 5). In order to compare the performance of the system of FIG. 5 with other prior art systems, a simulation of FIG. 5 as shown in FIG. 6 is presented below wherein the pump light from the Raman laser is only co-propagating, i.e., a system in which second WDM 122 is not present and aside from second FBG 120B, sensing fiber 126 has a free end.
The sensor system shown in FIG. 5 is also an example of a second order in-line Raman amplification setup which enables the amplification reach to be extended even further into the optical fiber as compared with the sensor system of FIG. 3. An example of this is described in Martin-Lopez, S. et al., “Brillouin optical time-domain analysis assisted by second-order Raman amplification,” Optics Express, volume 18, number 18, August 2010, pp. 18769-18778. In the system of Martin-Lopez et al., FBGs and pump diode-lasers are positioned at both ends of an optical fiber as part of a fiber optic sensing system, thereby creating an ultra-long Raman fiber laser (herein abbreviated as URFL).
In the sensing system of FIG. 5, the Raman gain for the probe pulse and the back-scattered light is supplied by the mode of the URFL. Dynamics of the URFL require a number of roundtrips of this mode for it to build up and supply substantial Raman gain. In such a setup, it is impossible to significantly modulate the Raman gain according to a desired temporal gain profile as was the case of in fiber optic sensing system 50 (FIG. 3). Specifically, in the sensing system of FIG. 5, the Raman gain cannot be reduced during the time of the probe pulse. In order to prevent the probe pulse power from exceeding the non-linear threshold, it is necessary to reduce the probe pulse power that is coupled to optical fiber 126, so that after Raman amplification, the probe pulse power will reach the maximum allowed power (i.e., the non-linear threshold) only some distance along the sensing fiber.
Reference is now made to FIG. 6, which is a graph of a simulation using the fiber optic sensing system of FIG. 5, showing the absolute value of the difference between two consecutive probe light measurements on a logarithmic scale, generally referenced 140, as is known in the art. Graph 140 includes an X-axis 142, showing distance in kilometers along an optical fiber (such an optical fiber 126 in FIG. 5), and a Y-axis 144, showing the power in dBm. The simulation as shown has been designed to exhibit disturbances between measurements along the optical fiber at the following distances: 15 km, 40 km, 65 km, 90 km and 115 km. Regarding the parameters of the sensor system of FIG. 5, in order to remain within the threshold of preventing non-linear phenomena, the peak power of the probe pulse was reduced to 100 microwatts. The second order Raman forward pump power for the forward propagating pump light was set to 1.45 watts and in order to enable single ended operation (for comparison purposes as explained above), the Raman backward pump power for the counter-propagating pump light was set to zero. This set of parameters ensures that the probe pulse will not exceed the 5 mW power non-linear threshold and also maintains the URFL mode power under the 400 mW threshold for spontaneous Raman scattering.
An envelope 146 of the noise power is shown, showing an initial decrease over the first 10-15 km, shown by an arrow 150, and then a significant increase in power peaking around 40 km after which there is a slow attenuation over the next 100 km. Shot noise 152 present in the optical fiber is shown along with a plurality of arrows 148A, 148B, 148C and 148D showing disturbances at 40 km, 65 km, 90 km and 115 km respectively. However, the disturbance at 15 km is masked by the shot noise and is not detectable using the sensor system of FIG. 5.
It is noted that the low power of the probe pulse might introduce additional noise in the optical fiber which is not considered in the simulation of FIG. 6. The simulation of the system of FIG. 5 as shown in FIG. 6 does not consider both forward propagating and backward propagating Raman pump light, however nevertheless, the system of FIG. 5 requires access to both ends of the optical fiber for installing the two FBGs which could be a significant drawback and preventing installation of such a system on an existing buried optical fiber cable, for example. While using second order in-line Raman amplification does increase the gain further into the fiber, which now peaks around 40 km, the configuration still suffers from a lack of full temporal control over the amplification. The fact that the first order Raman mode power is determined by the FBGs makes it impossible to control the power of the Raman gain on a timescale shorter than a roundtrip time, which was the case of the simulation shown in FIG. 4. A result of this lack of control over the power of the Raman gain is the low input power of the probe pulse, as implied by arrow 150 that leads to the poor SNR shown at the beginning of the optical fiber before the probe pulse is amplified to 5 mW. As mentioned, this is very noticeable in the probe pulse at 15 km wherein the signal power is comparable to the shot noise limit, substantially making the disturbance at 15 km unnoticeable and not detectable.
Prior art use of second order Raman amplification is shown in FIG. 5. This sensor system differs from the sensor system of FIGS. 1 and 3 in a few ways. One is that the optical fiber is spliced to an FBG at each end. The FBGs form a cavity at a wavelength for which the probe pulse wavelength is near the Raman scattering spectral peak. The cavity is pumped at both ends to form a URFL. The wavelength of the pump diode laser is chosen such that the cavity wavelength is close to the Raman scattering peak. As mentioned above, this architecture has a few drawbacks. First, the optical fiber must be accessed at both ends. This eliminates the use of this sensing system in harsh environments where electronics cannot be used, either because of electromagnetic noise, heat, extreme cold or other reasons. In applications that require several control units such as long pipelines and borders which may be hundreds if not thousands of kilometers long, the sensor system of FIG. 5 halves the distance between control stations or control points where detectors are to be installed. Furthermore, in the sensor system of FIG. 5, the first order Raman mode cannot be controlled independently from the second order Raman mode, thus the gain profile is determined by design and cannot be altered in case of manufacturing variance. Moreover, the gain profile cannot be changed dynamically to adjust to environmental conditions, regions of interest or variations in the power of the probe pulses and the back-scattered signals.